In recent years, full color recording systems for directly recording a still image photographed with a video camera and a still image on a television, a video tape recorder, a video disk, a computer, etc. on an image-receiving sheet are being developed. In particular, attention is now focused on a recording system in which a coloring sheet coated with a coloring material that is melted, vaporized, or sublimated by application of heat is superposed on an image-receiving sheet, and the coloring sheet is heated with a thermal head according to recording signals to transfer the coloring material to the image-receiving sheet, thus forming an image through adhesion, adsorption, or dye-fixing of the coloring material to the image-receiving sheet.
This recording system is generally characterized in that plain paper, a plastic film, or the like can be used as the image-receiving sheet since the coloring material on thc coloring sheet is melted, vaporized, or sublimated by application of heat.
However, in the case that a sublimable dye is employed as the coloring material, when plain paper, a plastic film, etc. is used as the image-receiving sheet, dye-fixing is, in particular, difficult to accomplish. As a result, there can only be obtained recorded images having low density and such images also have a defect of fading with time.
There has been therefore proposed a method in which a substrate is coated with a thermoplastic polyester resin or the like to provide an image-receiving layer (see, for example, U.S. Pat. No. 4,474,859). However, this proposed method involves such disadvantages that when plain paper is used as the substrate, penetration of the resin used in the image-receiving layer into the substrate occurs, and a coloring sheet cannot be brought into close contact with an image-receiving sheet at printing because of poor surface smoothness and cushioning properties of the image-receiving sheet, resulting in still insufficient image density and considerable unevenness of recorded images. Thus, image-receiving sheets of high quality are difficult to obtain by the above-proposed method. For improving the above problems, it is attempted to use as a substrate a so-called coated paper obtained by forming on a base paper a pigment coating layer comprising a pigment and a binder as main components. Although this substrate is improved in resin penetration and surface smoothness, it is still insufficient in cushioning properties, resulting in insufficient improvements in image density and unevenness of recorded images.
In the case that a plastic film is used as a substrate, recorded images free from unevenness and having high density can be obtained in some cases, because the surface smoothness is excellent, and some plastic films show good cushioning properties. However, because of high temperature at printing (a thermal recording head of a thermal transfer recording device is generally heated to 200.degree. C. or higher), there is a problem that the surface of the plastic film is heat deformed, likely leading to occurrence of remarkable curling. Furthermore, the prudction cost is high as compared with that in which paper is used.
JP-A-60-236794 proposes an image-receiving sheet comprising a base paper, an interlayer formed on the base paper and comprised of a thermoplastic polymer which provides surface smoothness and cushioning properties, and an image-receiving layer formed on the interlayer. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
The above image-receiving sheet, however, has been found to have the following problems. That is, the above proposed image-receiving sheet is generally produced by coating a solution of a thermoplastic resin, e.g., polyester resins or acetate resins, dissolved in a solvent on the interlayer and heat-drying the coating solution to form an image-receiving layer. If part of the solvent used remains in the image-receiving layer, the recorded image after the printing is inferior in storage stability (i.e., fading, oozing, etc. occur). If the drying is performed at a high temperature in order to completely remove the solvent, the interlayer comprised of a thermoplastic polymer undergoes deformation due to the high temperature to give poor appearance or impair surface smoothness and, as a result, recorded images free from unevenness and having high density cannot be obtained. On the other hand, if the drying is performed at a lower temperature in order to prevent deformation by heat, it takes much time, resulting in a very low productivity. In addition, since the solvent is harmful to human bodies and involves a fear of explosion, it is troublesome in handling and, further, it is expensive. Thus, there are problems from the standpoints of safety and cost.
In the case that the image-receiving layer is formed by use of an aqueous resin, it is superior in safety, etc. to those formed by use of a solvent-based resin. However, the former also requires a drying step which is accompanied by heat problems. Moreover, use of an aqueous resin is defective in that not only image-receiving sheets having poor surface gloss are merely obtained, but recorded images involve problems in storage stability because of inferior water resistance and moisture resistance.
In addition, as the method for forming an interlayer comprised of a thermoplastic polymer as described above, if a method in which a thermoplastic film is laminated on a base paper is employed, there are problems in occurrence of curling during the production and safety by a solvent. On the other hand, if a method in which a coating solution of a thermoplastic resin dissolved in a solvent is applied is employed, penetration of the solution into the base paper likely occurs, resulting in an insufficient improving effect in surface smoothness. Further, not only curling or unevenness likely occurs at drying, but there is a problem in safety by the solvent.